TM 55-1510-222-10 event    of    a    fire    or    overtemperature    in    the    engine compartment   (fig.   2-12).      The   main   element   of   the system is temperature sensing tubing, routed continuously throughout the engine compartment terminating  in  a  responder  unit.    The  responder  unit  is mounted  in  the  accessory  area  on  the  upper  left  hand engine  mount  truss,  just  forward  of  the  engine  firewall. The responder unit contains two sets of contacts: a set of  integrity  switch  contacts,  for  continuity  test  functions of the fire detection circuitry; and a set of alarm switch contacts,  which  complete  the  circuit  to  activate  the  fire warning   system   when   the   detector   (sensing   tubing) senses  an  overtemperature  condition  in  critical  areas around the engine.  The detector is dual functioning and responds  to  overall  "average"  temperature,  or  a  highly localized  "discrete"  temperature,  caused  by  flames  or hot gases.  Both the average and discrete temperatures are preset, and cannot be adjusted in the field. The sensor tubing consists of an outer tube filled with an inert gas, and an inner hybrid core that is filled with  an  active  gas.    The  gases  within  the  tube  form  a pressure barrier that keeps the contacts of the responder integrity   switch   closed   for   fire   alarm   continuity   test functions.  As the temperature around the sensing cable increases,  the  gases  within  the  tube  begin  to  expand. When the pressure from the expanding gases reaches a preset point, the contacts of the responder alarm switch close, activating the respective fire warning system. b.    Warning  System.    The  fire  warning  system consists of two lenses, placarded #1 FIRE PULL and #2 FIRE    PULL    located    in    the    T    handles    below    the glareshield,    two    MASTER    WARNING    annunciators located  in  opposite  sides  of  the  glareshield,  and  two responder units with sensors in the engine compartments.    If  the  detector  should  develop  a  leak, the loss of gas pressure would allow the integrity switch to open and signal a lack of detector integrity. c.    Testing.    Testing  system  integrity,  availability of power, and the annunciators, (#1 and #2 FIRE PULL and    MASTER    WARNING)    is    accomplished    by    two switches   located   in   the   copilot's   left   subpanel.      The switches  are  placarded  ENG  FIRE  TEST,  DET  -OFF  - EXT, LEFT and RIGHT.  When either (LEFT or RIGHT) switch  is  placed  in  the  DET  position,  electrical  current flows  from  a  5-ampere  circuit  breaker  placarded  FIRE DETR   located   in   the   overhead   circuit   breaker   panel, through the engine fire detector circuitry to the integrity switch contacts in the respective responder unit, causing the  respective  annunciators  to  illuminate.    If  the  circuit breaker opens, the system will not operate during a test, or activate the annunciators if the detector cable senses an   overtemperature   condition.      The   system   may   be tested either before, after, or in flight as desired. 2-26.  ENGINE FIRE EXTINGUISHER SYSTEM. a.      Description.      The   engine   fire   extinguisher system   is   comprised   of   a   supply   cylinder,   explosive squib, and valve located in each of the main gear wheel wells.    A  gage  calibrated  in  PSI  is  provided  on  each supply cylinder for determining the level of charge.  The extinguishing   agent   charge   level   should   be   checked during  each  preflight.    When  fired,  the  explosive  squib opens    the    valve,    releasing    all    of    the    pressurized extinguishing   agent   into   a   plumbing   network.      The plumbing network terminates in spray nozzles, strategically  located  in  the  probable  fire  areas  of  the engine compartment. b.  Operation.  Fire control T handles used to arm the   extinguisher   system   are   centrally   located   in   the instrument   panel,   immediately   below   the   glareshield. These controls receive power from the hot battery bus. The  fire  detector  system  will  indicate  an  engine  fire  by illuminating  the  MASTER  WARNING  annunciators  on the glareshield and the respective #-1 or #2 FIRE PULL annunciators  in  the  fire  control  T  handles.    Pulling  the fire control T handle will electrically arm the: extinguisher system and close the firewall shutoff valve for    that    particular    engine.        This    will    cause    the annunciator  in  the  PUSH  TO  EXTINGUISH  switch  and the respective #1 and #2 FUEL PRESS annunciators in the  warning  annunciator  panel  to  illuminate.    Pressing the  lens  of  the  PUSH  TO  EXTINGUISH  fire  switch  will fire  the  squib,  expelling  all  the  agent  in  the  cylinder  at one time.  A hinged plastic guard covers the PUSH TO EXTINGUISH     fire     switch     to     prevent     inadvertently actuating  the  fire  extinguish  switch  squib  circuit.    The respective   caution   annunciator,   #1   and   #2   EXTGH DISCH  on  the  caution/advisory  annunciator  panel  and the  MASTER  CAUTION  annunciator  on  the  glareshield will illuminate and remain illuminated, regardless of the master switch position, until the squib is replaced. c. Testing.    The  test  switches  located  in  the copilot's left subpanel, are placarded ENG FIRE TEST, DET - OFF - EXT, LEFT' and RIGHT, and provide a test of  the  fire  detection  and  extinguisher  circuitry.  When either of the switches is placed in the EXT position, the corresponding PUSH TO EXTINGUISH, SQUIB OK and EXTGH DISCH 2-26